To: atm{at}shore.net
From: mdholm{at}telerama.com
Reply-To: mdholm{at}telerama.com


Dwight Elvey wrote,

>Then, you could use a spreader to get a more uniform laminar
>flow across the face of the mirror.

If the air is uniform in temperature, it doesn't matter if it is turbulent.
The pressure variation implied by the sort of turbulence you would get a
couple of inches from a fan is too small to make an appreciable refractive
index effect.  The point of using the fan is two fold: 1. Increase cooling
rate of the mirror to bring it to equilibrium faster.  2. Break up the
layer of (usually) warm air that tends to form near the front of the mirror
and blow it away.  Turbulent air flow works as well for this as laminar.

It is the temperature gradients in air that cause bad "tube
currents".  Small temperature differences make fairly large refractive
index changes in air. These gradients can be simply blown out of the
optical path.  The air, turbulent or not, that replaces them will be much
more thermally uniform, unless the air is coming from a source with strong
temperature differences.

Turbulence isn't the big enemy, temperature differences are.  I know it
sounds counterintuitive, but that is exactly what the experiments reported
in S&T show.

As for the pattern of air flow, the S&T experiments used a fan blowing
across not at the mirror face.  (Note: this is the front face, not the rear
as shown in Kriege and Berry.)  With this geometry, I don't think the flow
pattern Dwight describes will occur.

Mark Holm
mdholm{at}telerama.com

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